Zippo K XV solid brass lighter K represents the month of November XV for the year 1999 and a brass no 5 Lighter both new flint and fuel left uncleaned to give it a rustic appeal but if you would rather clean it I recommend Lemon Juice & Baking Soda (Best for Deep Cleaning)
Method: Combine 1 tablespoon of lemon juice with 1-2 teaspoons of baking soda to make a paste.
Application: Apply the paste to the brass, allowing it to sit for 30 minutes to an hour.
Finish: Rub with a soft cloth to remove grime, rinse with warm water, and dry completely.